For the synthesis of polysilanes, the Wurtz coupling method of condensing dihalosilanes with the aid of alkali metals has been most widely used partially because the starting reactants are readily available compared with other methods. This method is applicable to the synthesis of polyorganosilanes soluble in organic solvents from difunctional diorganodihalosilanes and alkali metals, but the polyorganosilanes are obtained only in low yields due to formation of solvent-insoluble components. Probably these solvent-insoluble components are formed directly by radical polymerization or radical polymerization causes polysilane side chain moieties to crosslink into insoluble components although the polymerization mechanism associated with the Wurtz coupling reaction is complex and not well understood.
The inventors have found two main causes of the formation of insoluble components. First, insoluble components are formed during Wurtz coupling reaction. Secondary, insoluble components are also formed during the post-treatment of dissolving away a salt (such as sodium chloride), which is produced during Wurtz coupling reaction, with water. More particularly, the post-treatment includes the addition of alcohol for deactivating the alkali metal and the addition of water for washing away the salt. The formation of insoluble components during the post-treatment is presumed due to crosslinking occurring when side chain components are degraded and condensed into siloxanes through alkoxylation/silanol formation due to addition of alcohol/water. Then the majority of insoluble components are formed in the steps of deactivating the excess alkali metal used in Wurtz coupling reaction and dissolving away the salt produced during Wurtz coupling reaction. In washing away the salt (such as sodium chloride), the insoluble components tend to form an emulsion, disturbing separation between the organic solvent and water layers. It is thus time-consuming to purify the desired solvent-soluble polysilane in the organic layer.